Syngamus trachea (Fig. 1). The adults of this so-called red tracheal worm measure 8 mm (males) in length and 20 mm (females), respectively, which live in permanent copulation attached at the tracheal wall, thus offering a Y-shaped aspect. Both sexes suck constantly blood at different places and thus appear reddish in shape. The females excrete the thick-walled 70–100 μm × 45–50 μm-sized eggs in an unembryonated stage (containing a morula of about 16 cells). The egg shell is characterized by thickenings at both poles (Fig. 1). The larva 3 is formed inside the egg shell within 1–2 weeks. If snails, earthworms, insects, or their larvae ingest such eggs, they may become accumulated there (larvae 3 may hatch, too). If birds ingest larvae 3 containing eggs or swallow such infested intermediate hosts, the larvae penetrate into the wall or directly into the body cavity. They reach the lung via the bloodstream or directly by entering the lung from the body cavity. From there they migrate into the lumen of the trachea, where they copulate and reach maturity within 1 week (Fig. 2). However, another week is needed until the first eggs are excreted.

Syngamus (syn. Cyathostoma) bronchialis. This species parasitizes in the trachea and bronchioles of geese, ducks, and free-living birds. The males reach a length of about 6 mm, while the females are considerably longer (up to 3 cm). The adults do not live in permanent copulation, and the males appear whitish, while the females appear even dark red due to their permanent blood sucking. The further development is similar to that of S. trachea (Fig. 1).

Syngamus (syn. Mammomonogamus) laryngeus occurs in the larynx of cattle and buffaloes in Asia, South and Central America, Sri Lanka, and the Philippines.

Syngamus (syn. Mammomonogamus) nasicola is found in the nose cavities of ruminants in Africa and South and Central America.

Mammomonogamus (syn. Syngamus) species of animals are also found in cats and in humans.